This invention relates to an optical spatial filter device and an image sensing apparatus using this device. More particularly, the present invention relates to improvements in an optical spatial filter device for limiting or attenuating specific spatial frequency components (e.g., high-frequency components) in the image of a subject when a still picture or moving picture is sensed using an image sensing apparatus.
In an image sensing apparatus using an image sensing device such as a CCD, the image of a subject that has been formed on the image sensing device is subjected to spatial sampling. Consequently, when the image of a subject which includes frequency components greater than half the sampling frequency (i.e., greater than the Nyquist frequency) is sensed, aliasing distortion occurs and appears as moirxc3xa9 in the image obtained from the image sensing device. The result is a pronounced decline in image quality.
In an image sensing apparatus of this kind, therefore, the general practice is to suppress the occurrence of moirxc3xa9 by utilizing a birefringence plate comprising quartz or the like disposed between an image sensing lens and the image sensing device, as disclosed in the Japanese Patent Publication (KOKOKU) No. 58-14116. Such a birefringence plate is referred to as an optical low-pass filter and separates incident light into ordinary light and extraordinary light by the action of birefringence. As there is an offset between the ordinary light and extraordinary light, the image appears blurred and frequency components above the Nyquist frequency are limited or cut off.
However, the effects of an optical low-pass filter that utilizes the birefringence of a quartz crystal or the like act in only one dimension. Accordingly, in an image sensing apparatus using the conventional image sensing device, it is required that a plurality of birefringence plates be combined in order to realize a two-dimensional spatial-frequency limiting characteristic. Placing a plurality of birefringence plates in an optical path is disadvantageous in that this expedient lengthens the optical path of the image sensing apparatus.
Further, it is known that the Nyquist frequency for one color and the Nyquist frequency for all pixels differ in an image sensing apparatus that uses a single-plate color image sensing device (such as a single plate CCD) on which a color filter array is disposed.
Accordingly, to obtain a color image having good resolution using a color image sensing apparatus, it is required that the spatial-frequency limiting characteristics of the applied optical low-pass filters of quartz or the like be made varied so that the characteristic for the image of a luminance signal will be different from that for the image of a color signal. However, in order to change the spatial-frequency limiting characteristics of the optical low-pass filters in the prior art, a plurality of optical low-pass filters having different spatial-frequency limiting characteristics are provided to be interchanged. The filters can only be interchanged by insertion and removal. A practical problem which arises is that it is difficult to momentarily interchange low-pass filters by insertion and removal. This means that it is difficult to change the spatial-frequency limiting characteristic in real-time.
An object of the present invention is to provide an optical spatial filter device and an image sensing apparatus using this device, wherein it is possible to vary a two-dimensional spatial-frequency limiting characteristic in real-time through a comparatively simple arrangement.
Another object of the present invention is to provide an image sensing apparatus and an optical spatial filter device for obtaining a target spatial frequency characteristic by rotating an optical image along a circle or ellipse of a predetermined radius relative to an image sensing device.
According to an aspect of the present invention, the path of rotation is closed.
According to another aspect of the present invention, the path of rotation is circular or elliptical.
According to yet another aspect of the present invention, rotation is realized by a glass plate with parallel, flat surfaces.
According to yet another aspect of the present invention, rotation is performed by rotating the image sensing device.
According to yet another aspect of the present invention, rotation is performed an integral number of times during an exposure interval of the image sensing device.
According to yet another aspect of the present invention, rotation is performed at uniform angular velocity.
According to yet another aspect of the present invention, radius of rotation or radius of curvature is decided in correlation with a frequency to be trapped.
Another object of the present invention is to provide an image sensing apparatus and an optical spatial filter device for obtaining a target spatial frequency characteristic by moving the focal-point position of an optical image back and forth relative to an image sensing device in parallel with the direction of the optic axis.
A further object of the present invention is to provide an image sensing apparatus and filter for obtaining a target spatial frequency characteristic by superposing two periodic motion modes.